DIFPERENTIATION OF INDIVIDUALS AND ADAPTATION III 



When the causes which produce varieties have been at work 

 long enough to eliminate the intermediate forms which are often 

 found connecting the varieties, and to secure a close adaptation 

 of the varieties to the environment, the term species is applied to 

 what were formerly called varieties. Species thus merely rep- 

 resent the further progress of individual differentiation and adap- 

 tation to the different modes of life which give rise to variation in 

 individuals — that is, to varieties. If a mutation is of sufficient 

 importance to serve as a basis of distinction and breeds true, 

 a new species may be produced in one generation, without any 

 intergrades. A species of animals may again split up by the 

 action of the forces mentioned (and other conditions which 

 have not been mentioned) into new varieties and finally into 

 new species. It is believed that the present diversity of animal 

 and plant life has come about from a much more limited num- 

 ber of kinds of ancestors by a method essentially such as that 

 described above. The student will realize that in nature there 

 are only individuals, no two of which are exactly alike. There 

 is really no such thing in nature as a species. This is purely a 

 mental concept of our own. 



Varieties of the same species usually cross freely. Their 

 offspring are usually fertile. The individuals of different spe- 

 cies as a rule cross .less freely and when they do cross the term 

 hybrid has been applied to their offspring. Such hybrids are 

 often sexually infertile. In recent years geneticists class as 

 hybrid the crosses of subspecies as well. 



The genus is related to species somewhat as the species to 

 the varieties which compose it. A genus embraces those kin- 

 dred species which show a high degree of relationship among 

 themselves. The characters which serve to distinguish differ- 

 ent genera are more fundamental than those by which we 

 recognize varieties or species, and argue a more extended time 

 in the differentiation of genera than is required for species. 



Other terms, as families, orders, classes, phyla, are used to 

 denote the still more extensive and comprehensive division 

 of the animal kingdom. 



143. Illustration of Classification. — The domestic cat as a 

 species has many varieties or breeds, as the maltese, manx, 



